ortho-Phosphoryl stabilized hypervalent iodosyl- and iodyl-benzene reagents

Article abstract of DOI:10.1016/j.tetlet.2005.05.111

The synthesis and reactivity of a new IBX analogue (2-iodylphenyl)diphenyl- phosphine oxide 10 is described herein along with its analysis by single crystal X-ray diffraction.

Full text of DOI:10.1016/j.tetlet.2005.05.111

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5187–5190
ortho-Phosphoryl stabilized hypervalent iodosyl- and
iodyl-benzene reagents
Bindu V. Meprathu, Michael W. Justik and John D. Protasiewicz*
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA
Received 16 May 2005; revised 25 May 2005; accepted 25 May 2005
Available online 21 June 2005
Abstract—The synthesis and reactivity of a new IBX analogue (2-iodylphenyl)diphenyl-phosphine oxide 10 is described herein along
with its analysis by single crystal X-ray diffraction.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
O
I
O
S
I
O
S
I
O
S
O
O
X
X
Hypervalent iodine compounds1 continue to find impor-
tance as oxo- and nitrene-sources for metal catalyzed
aziridinations2 and epoxidations, respectively.3 The prin-
cipal reagents used for these important transformations,
iodosylbenzene (PhI@O) and (tosyliminoiodo)benzene
(PhI@NTs), however, are dominated by strong inter-
molecular secondary bonding interactions, which render
these compounds virtually insoluble in all non-reactive
media.4 This fact inhibits the ability to enhance catalytic
activity, perform careful mechanistic studies, grow crys-
tals for single crystal X-ray diffraction studies, and to
spectroscopically observe intermediates in oxidative
processes.
O
O
O
1a,b
2
3a,b
CF3
O
I
O
O
I
O
O
S
I
O
O
O
O
O
R
HN
R
E
R
5
6
4
a: X = O
b: X = NTs
Previously, we have found success by the incorporation
of ortho-tert-butylsulfonyl groups into these two parent
reagents that introduce intramolecular secondary bond-
ing. The strategic location of a suitable negatively
charged donor atom offers the electrophilic iodine center
the opportunity to coordinate to a proximally located
intramolecular oxygen atom rather than to a negatively
charged donor atom of another molecule that might re-
sult in coordination polymerization. Some examples
from our group (1–3) are shown in Chart 1.5 Whereas
iodosylbenzene is practically insoluble in chloroform, a
0.08 M solution of 1a can be prepared. Similarly, the
Chart 1.
poor solubility of (tosyliminoiodo)benzene is enhanced
50-fold in CHCl3 with the introduction of an ortho-
tert-butylsulfonyl group in 1b. Such reagents have been
used successfully for homogeneous metal-catalyzed azir-
idinations and epoxidations.
More recently, this approach of integrating donor
groups into hypervalent iodine compounds has led to
the successful development of practical replacements
for the synthetically useful oxidant IBX (1-hydroxy-
1,2-benziodoxol-3(1H)-one-1-oxide), namely the IBX-
amides 4,6 IBX-esters 5,7 and IBX-sulfonamides and
sulfonate esters 6 (E = NH or O).8 Like 1–3, these
reagents exhibit improved solubility compared to their
Keywords: Iodosylbenzene; Iodylbenzene; Hypervalent iodine; Poly-
coordinated iodine.
*
Corresponding author. Tel.: +1 216 3685060; fax: +1 216 3683006;
e-mail: protasiewicz@case.edu
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.111

5188
B. V. Meprathu et al. / Tetrahedron Letters 46 (2005) 5187–5190
simpler unsubstituted analogues. Polymer supported
IBX-analogues have also been reported.9
treatment with peracetic acid followed by aqueous
NaOH,15 yielded a mixture of unreacted 8 and triphe-
nylphosphine oxide. An alternative method involving
initial reaction of 8 with chlorine gas to generate ArICl2
(Ar = 2-Ph2P(O)C6H4), followed by hydrolysis with
base,16 was unsuccessful due to failure to obtain the
ArICl2 intermediate. It was found, however, that if the
material produced from the action of peracetic acid on
8 was dissolved in CHCl3 and washed with cold H2O
until the odor of acetic acid was eliminated, evaporation
of the chloroform followed by trituration with ether
would give a product consistent with 9.17 Elemental
analysis of this white solid from two different prepara-
tions matched the expected composition along with a
The importance of secondary bonding interactions with
regard to polycoordinated iodine chemistry has also
been proven in developing chiral hypervalent iodine
oxidants,10 for directing the self-assembly of amino acid
derived benziodoxoles,11 and for isolating novel com-
plexes of diphenyl-k3-iodanes with crown ethers.12
Given this recent excitement for these novel iodyl species
as safe alternatives to IBX, we wish to report our work
in which the ortho-tert-butylsulfonyl group of 1a and 2
are replaced by the diphenylphosphoryl unit as a source
of intramolecular secondary bonding. Herein, we report
the preparation and characterization of a new potential
IBX analogue (2-iodylphenyl)diphenylphosphine oxide,
10.
1
fractional amount of CHCl3 (0.86). H NMR analysis
indicated a downfield shift of the signal for the proton
ortho to the iodine atom from d 8.01 in 8 to d 8.78 in
9, consistent with what is observed for other hypervalent
iodine compounds. Analysis by 31P NMR spectroscopy
gave a correspondingly downfield shift from d 33.7 in 8
to d 38.4 in 9.
2. Results and discussion
The requisite precursor to the title compounds, (2-iodo-
phenyl)diphenylphosphine oxide, 8, has been prepared
by ortho-directed lithiation of Ph3P@O followed by
reaction with elemental iodine.13 The maintenance of
low temperatures (ꢀ25 ꢁC) over an extended period of
time (72 h) during the lithiation step, however, made
the two step process shown in Scheme 1 more attractive.
(2-Bromophenyl)diphenylphosphine 7 was first prepared
by using the method of Tunney and Stille14 (62%), and
then converted to 8 by metal–halogen exchange and sub-
sequent reaction with iodine (71%).
Attempts to crystallize the putative iodosylbenzene
9 from a mixture of chloroform and benzene yielded
X-ray diffraction quality single crystals. Analysis of the
data, however, revealed the identity of the material as
the iodyl derivative 10Æ2H2O (Fig. 1). As the precursor
iodobenzene 8 was highly crystalline, an X-ray diffrac-
tion was also performed on this substance to allow com-
parisons to 10 (Fig. 2).
Several structural features of 10 merit comment, in par-
ticular, the features that differ from those found in the
Oxidation of 8 to a fully characterized iodosylbenzene 9
that could be compared to iodosylbenzene 1a proved to
be a challenging task. Subjecting 8 to the conventional
protocol for the preparation of iodosylarenes, namely,
Br
Br
Ph2P-TMS
(CH3CN)2PdCl2
Ph2P
I
benzene, 60-70 oC
100 h
7
1. n-BuLi, THF, -78 oC
2. I2
O
I
Ph2P
8
NaOCl (5.25% aq.)
TBA-Br
peracetic acid, 32%
H2O/CH2Cl2, rt, 1 h
O
O
I
O
O
I
"
"
O
Ph2P
Ph2P
disproportionation
Figure 1. X-ray crystal structure of iodylarene 10. Selected distances
˚
I1–C1 2.153(15), P1–O1 1.498(11), P(1)–C(1) 1.808(16), I1ꢁ ꢁ ꢁO20 2.564;
C1–I1–O3 94.1(6), O3–I1–O2 103.7(6), C1–I1–O2 93.2(6), O(1)–I(1)–
O(3) 167.9(5).
- 8
[A] and angles [ꢁ]: I1–O3 1.789(12), I1–O2 1.818(10), I1–O1 2.603(11),
10
9
Scheme 1. Synthesis of 9 and 10.

B. V. Meprathu et al. / Tetrahedron Letters 46 (2005) 5187–5190
5189
2 leads to an infinite bifurcated polymeric structure.
While one can consider 10 to be dimeric in the solid
state, as we have chosen to show in Figure 2, close con-
tacts of the iodine atoms with neighboring iodobenzene
ring (but not with the same molecule depicted in Fig. 2)
indicate that there are most likely significant Iꢁ ꢁ ꢁarene
interactions as well. This interaction is trans to O2 and
the I1–O1 vector is parallel to this aromatic ring placing
˚
the iodine center at a distance of 3.47 A above this
neighboring plane of carbon atoms. The iodine atom
is not located above the centroid of this ring, and is
˚
nearly above C5 of this ring (3.57 A). This geometry
˚
places O1 3.76 A above C3 of the same ring. The crystal
structure of MesINTs (Mes = 2,4,6-Me3C6H2) revealed
a similar Iꢁ ꢁ ꢁarene interaction with a Iꢁ ꢁ ꢁcentroid dis-
18
˚
tance of 3.46 A. In summary, the packing diagram of
10 is complex, and the presence of intermolecular
Iꢁ ꢁ ꢁO secondary bonds, Iꢁ ꢁ ꢁarene interactions, and
hydrogen bonding makes for a tightly held network
polymer.
˚
Figure 2. Single crystal X-ray structure of 8. Selected distances [A] and
The genesis of 10 from efforts to crystallize 9 is most
likely the result of a disproportionation reaction. It is
well known that iodosylbenzenes can undergo dispro-
portionation19 to give an equivalent of the reduced iodo-
benzene and an equivalent of iodylbenzene. This
tendency has been suggested to be more prominent
in substituted compounds.20
angles [ꢁ]: I1–C1 2.115(6), P1–O1 1.474(4), P1–C2 1.811(5); O1–P1–C2
114.7(7).
structural analyses of 2 and 8. First, as anticipated, there
is a close contact of the phosphoryl oxygen atom with
˚
the iodine atom. This Iꢁ ꢁ ꢁO3 distance of 2.612 A is com-
˚
parable to the value of 2.693(2) A found in sulfone
derivative 2, and significantly shorter than the Iꢁ ꢁ ꢁO3
A more direct path to the highly stable iodylbenzene 10
was realized by treatment of 8 with aqueous sodium
hypochlorite under phase transfer21 conditions (71%).22
Compound 10 is insoluble in chloroform, acetonitrile,
benzene, and ether.
˚
distance of 3.291 A determined for 8. Intramolecular
secondary bonding in 10 induces a roughly coplanar
geometry for atoms O3, P1, C2, C1, and I1 (mean devia-
˚
tion from best plane 0.12 A). However, careful analysis
of this five atom array reveals that both the I1 and O3
atoms are slightly displaced from either side of the best
plane of the six aromatic carbons of the iodobenzene
Preliminary investigations into the oxidation behavior
of 9 and 10 have found them to be active oxidants. Tri-
phenylphosphine is rapidly and quantitatively converted
at room temperature to triphenylphosphine oxide in
d-chloroform in the presence of 9 or 10. Similarly,
methyl p-tolyl sulfide is converted to the corresponding
sulfoxide in moderate yields (41% and 30%, respectively)
after protracted (2 days) exposure to 9 or 10 in d-chloro-
form at room temperature. Thus, the present materials
appear to be less effective than the IBX-ester analogue
5 (R = i-Pr) for selective oxidations.7b An exploration
of further oxidation chemistry of these compounds is
currently underway.
˚
ring by ꢀ0.29 and +0.27 A, respectively. Compound 2
shows displacements of the I and O3 atoms from the
same side of the least squares plane of the six aromatic
carbons of the iodobenzene ring by +0.39 and
˚
+0.50 A, respectively. Such minor displacements are
possibly induced by the close C6–Hꢁ ꢁ ꢁO1 contacts of
˚
2.38 and 2.39 A that occur in 10 and 2. The I–O1 and
I–O2 bond lengths for 10 are nearly identical to those
determined for 2, as are the bond angles involving the
iodyl unit. One of the two water molecules (O4) within
the lattice of 10 appears to engage in some degree of
hydrogen bonding to O1 of the iodyl unit (hydrogen
atoms of O4 located and refined) with a O1ꢁ ꢁ ꢁH(O4) dis-
˚
tance of 2.25 A. By contrast, O1 for 2 is found to partici-
Acknowledgments
pate in hydrogen bonding to a dichloromethane
molecule found in the crystal (O1ꢁ ꢁ ꢁH distance of
We wish to gratefully acknowledge the Petroleum
Research Fund and Case Western Reserve University
for support of this work.
˚
2.55 A). The hydrogen atoms of the second molecule
of water in 10 were not located with confidence, leaving
open the possibility of other hydrogen bonding
interactions.
Supplementary data
Molecules of 10 are aggregated in the solid state to form
dimers held together by intermolecular Iꢁ ꢁ ꢁO2 second-
Cystallographic data for the structural analyses of 8 and
10 have been deposited with the Cambridge Crystallo-
graphic Data Centre (CCDC Nos. 271725 and
271726). Copies of this information can be obtained free
˚
ary bonds (2.57 A), comparable in length to the shorter
of the two intermolecular Iꢁ ꢁ ꢁO2 secondary bonds found
in 2. The presence of these two Iꢁ ꢁ ꢁO secondary bonds in

5190
B. V. Meprathu et al. / Tetrahedron Letters 46 (2005) 5187–5190
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17. Compound 9: A solution of 8 (0.505 g, 1.30 mmol) and
tetra-n-butylammonium bromide (25 mg) in CH2Cl2
(20 mL) was stirred vigorously with 50 mL of 5.25%
aqueous sodium hypochlorite (Cloroxꢂ) for a period of
1 h and then allowed to stand overnight. Filtration of the
resulting crystals in the organic layer gave 9 (0.403 g,
1
71%); mp 205 ꢁC (exp. dec.); H NMR (DMSO-d6) 8.48
3. For some examples of oxo-transfer see: (a) Palucki, M.;
Pospisil, P. J.; Zhang, W.; Jacobsen, E. N. J. Am. Chem.
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(dd, 1H), 7.96 (t, 1H), 7.54–7.82 (m, 14H); 13C NMR
(DMSO-d6) 138.9, 138.2, 137.3 (d, JCP = 10.3 Hz), 137.2
(d, JCP = 42.8 Hz), 136.4, 135.0, 134.2, 133.3 (d, JCP
93.2 Hz), 133.2 (d, JCP = 7.2 Hz), 127.7 (d, JCP = 4.6 Hz);
=
31P d 39.5.
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22. Compound 10: A solution of peracetic acid (2 mL, 32% in
dilute acetic acid) was added to 8 (0.602 g, 1.48 mmol) and
stirred overnight at room temperature (a solution was
obtained within 1 h). The solution was concentrated in
vacuo to a moist, white solid. After trituration with water
followed by ether, the solid was dissolved in chloroform
(20 mL) and further washed with water (3 · 10 mL).
Concentration of the organic phase afforded 10; mp
105 ꢁC; 1H NMR (CDCl3) d 8.78 (d, J = 7.8 Hz, 1H), 7.94
(m, 1H), 7.4–7.9 (m, 12H); 13C NMR (DMSO-d6) d 154.5,
132.9, 132.6, 132.1, 131.8, 131.1, 131.0, 129.7, 129.0, 128.8,
122.5; 31P NMR (CDCl3) d 38.5; HRMS 403.9808, calcd
403.9827; Anal. Calcd for C18H14IO2PÆ0.86CHCl3: C,
43.32; H, 3.64. Found: C, 43.36; H, 3.31.